1. Field of the Invention
This invention relates generally to detectors for gas chromatography, continuous emission monitoring, and other types of devices used for field sampling of volatile organic compounds and the like. More specifically, the invention relates to a halogen specific detector and a direct connection between the halogen specific detector and a photoionization detector.
2. Description of the Related Art
A photoionization detector (PID) is a well known detector which generates a current due to photoionization of the analyte passing through the device. In the PID, the analyte absorbs radiation, typically in the ultra violet (UV) region of the electromagnetic spectrum, which causes the analyte to ionize into an electron and positive ion pair. Within the PID two or more electrodes are present which provide an extraction field and permit quantitation of the generated current. The light source can consist of UV generating lamps (DC, AC, inductively, or capacitively coupled or otherwise modulated power to create UV radiation passing through one or more windows which serve to limit the band of radiation passing into the PID), or can consist of a windowless design in which UV radiation at frequencies above that which can be transmitted through a window is utilized as the radiation source. The electrodes are positioned to minimize UV light from striking them, generating photoelectrons which can be a source of unwanted background current and noise. Additional electrode(s) can be used to serve as guard electrode(s) which serve to minimize leakage currents between the sensing electrodes.
A halogen specific detector is a thermionic device related to the work done by Rice (U.S. Pat. No. 2,550,498), and Roberts (U.S. Pat. No. 2,795,716). The Rice patent entitled "Method and Apparatus for Detecting Vapors and the Like" discloses directly heated anodic and cathodic structures, with the anodic structure being activated with an alkali containing salt. According to Rice, the precise theory of operation is not completely understood. The mechanism proposed by Rice indicates that alkali sensitizes a heated anode structure. The collision of chlorine (neutral) with the alkali results in a large current of positive alkali ions being emitted with the energy being supplied by the formation of an alkali halide on the anode surface. It appears that the response of the Rice detector functions by a stimulated alkali emission from an anodic structure. The Roberts patent discloses a device having similar principles of operation as Rice, and also discloses a central core around which one of the electrode structures is placed.
The response of the halogen specific detectors of Rice and Roberts have problems with stability, excessive broadening of chromatographic peaks, and sensitivity to the type and class of compound containing the halogen. With those type of detectors, the compounds oxidized at the heated surfaces, i.e., the anodic and/or cathodic structure. Therefore, these detectors were unreliable because of differences in oxidation rates of different types and/or classes of compounds (i.e., the differences in molecular structure), resulting in different efficiencies of generating free halogens. However, these types of detectors were useful for leak detection and similar uses where analysis time was relatively unimportant, and for applications where it was relatively unimportant to detect and distinguish several compounds from one sample.
Rice and Roberts are in direct contrast to the detectors proposed by Arimoto, Fujii, and Jimba regarding a (positive) surface ionization detector (J. Chromat., 355(1986)375-382, Anal. Chem., 62 (1990)107, and European Patent Application WO 86/06836), and that of Coulson (U.S. Pat. No. 5,019,517).
Coulson (U.S. Pat. No. 5,019,517) discloses a halogen specific detector in which the sensor's cathode is required to be "substantially alkali free", and requires the use of a temperature sensor in either the pyrolysis body or as part of the probe assembly, with temperature control being maintained by monitoring the temperature of the sensor. Attempts to operate a halogen specific detector as described in U.S. Pat. No. 5,019,517 utilizing high purity alumina structures or quartz structures (devoid of alkali metals) have failed to generate a reproducible response to halogens, in the absence of an alkali glass ceramic for the anode structure (as taught by Rice and Roberts).
In the past, photoionization detectors (PIDs) and electrolytic conductivity detectors (EICDs) have been connected in series to allow use of the same sample twice and to reduce the time required to do the analysis of specific compounds. Additionally, the PID and EICD have different abilities to detect certain types of compounds, and it is desirable to use each type of detector to detect and quantitate samples. Serial connection of the PID-EICD for independently mounted detectors was through a transfer line (typically heated to prevent condensation) and routed from the outlet of the PID into the GC oven and then into the EICD inlet port (or other alternate connector). Serial connection has limitations because of leaks between connections, cold spots (condensation of analyte), adsorption of analyte on active sites, reaction of analyte on reactive materials within the transfer line, and the occupation of two detector mounting sites (in a two detector gas chromatograph)-preventing the use of a third detector.
More recently, electrolytic conductivity detectors (EICDs) have been directly coupled to photoionization detectors (PIDs as is disclosed by U.S. Pat. No. 4,805,846 (Hall).
However, halogen specific detectors have been used as stand-alone detectors only. They have not been connected serially or in tandem with other detectors such as PIDs.